Ceramic honeycomb structures are used in vehicular exhaust systems to reduce pollutants. Such structures generally comprise a network of interconnected web walls that form a matrix of elongated, gas-conducting cells which may be square, octagonal or hexagonal in shape, for example. The network of web walls is surrounded by a cylindrical outer skin that is integrally connected to the outer edges of the web walls to form a can-or oval-shaped structure having opposing inlet and outlet ends for receiving and expelling exhaust gases through the matrix of cells.
Such ceramic honeycomb structures may be used as either particulate filters in the exhaust systems of diesel-powered automobiles or other equipment, or as automotive catalytic converters. When used as particulate filters, the open ends of the cells on the inlet and outlet ends of the structure are preferably plugged in “checkerboard” fashion such that exhaust gases entering the inlet end of the structure must pass through the porous, ceramic web walls before they are allowed to exit the open ends of the cells at the outlet end of the structure. When used as catalytic converters, the cells remain unplugged so that the exhaust gases may flow directly through them, and the cell walls are coated with a precious metal catalyst containing platinum, rhodium, or palladium for example. After the web walls reach a required light-off temperature, the catalyst impregnated over the web walls oxidizes CO2, and disassociates NOx into N2 and O2. Both applications of ceramic honeycomb structures are important in reducing pollutants that would otherwise be expelled into the environment.
Such ceramic structures may be formed by extruding a paste-like, ceramic precursor to cordierite, mullite, silicon carbide, or aluminum titanate through a die to simultaneously form the network of web walls along with the integrally-connected outer skin. The resulting extruded, green body is cut, dried and moved to a kiln which converts the green ceramic body into a fired ceramic body. The fired body may be then either plugged in the aforementioned pattern to form a diesel particulate filter, or subjected to a catalyst wash coat in order to impregnate the walls of the flow-through cells with the catalyst.
Unfortunately, during the extrusion, handling and firing procedures, internal damage can occur within the ceramic substrate which can compromise the performance of the body in removing pollutants from the automotive exhaust system where it ultimately resides. Such damage can include cracks oriented along the axis of rotation of the structure and cracks transverse to this axis, referred to hereinafter as axial cracks and “ring-off” cracks. Still other damage is manifested by a localized separation between the network of web walls, and the outer skin of the structure. Finally, external hairline cracks on the surface of the structure can occur, or other strength-compromising scratches and deformities.
Methods for testing various manufactured parts for discontinuities are also known in the prior art. Such methods include x-ray inspection and CT scans. However, such x-ray inspections are insensitive to the internal cracks which may exist within honeycomb ceramic structures unless the defect is larger than a certain size. Even when the defect is sufficiently large to be detected, the x-ray image must be examined carefully for fine details in order to discern such defects. The time to completely inspect one honeycomb structure can take hours, which is far too long to be used in connection with a practical manufacturing process. Other techniques based on the same principle as an x-ray inspection, such as laminography and tomography suffer from the same drawbacks, in that they require far too much time and effort to be able to effectively and reliably detect cracks and other discontinuities within a time frame suited to a practical manufacturing process.
Clearly, what is needed is a method for inspecting ceramic honeycomb structures which is capable of quickly and reliably detecting the presence or absence of such discontinuities as axial or “ring-off cracks”, skin separations, hairline cracks on the exterior, or other deformities or faults that could seriously compromise the function of the ceramic structure in an exhaust system. Ideally, such a method would be quick, non-invasive and well-suited for incorporation into standard manufacturing processes. Finally, it would be desirable if such a method were applicable both to green or fired ceramic structures so that the inspection method could be used both to obviate the need for firing defective green bodies, as well as to provide a final check as to the finished, fired product.